This optional key is used as a hint to launchctl(1) that it should not submit this job to launchd when loading a job or jobs. The value of this key does NOT reflect the current state of the job on the running system. If you wish to know whether a job is loaded in launchd, reading this key from a configuration file yourself is not a sufficient test. You should query launchd for the presence of the job using the launchctl(1) list subcommand or use the ServiceManagement framework's SMJobCopyDictionary() method.
Note that as of Mac OS X v10.6, this key's value in a configuration file conveys a default value, which is changed with the [-w] option of the launchctl(1) load and unload subcommands. These subcommands no longer modify the configuration file, so the value displayed in the configuration file is not necessarily the value that launchctl(1) will apply. See launchctl(1) for more information.
Please also be mindful that you should only use this key if the provided on-demand and KeepAlive criteria are insufficient to describe the conditions under which your job needs to run. The cost to have a job loaded in launchd is negligible, so there is no harm in loading a job which only runs once or very rarely.